Carburized filament and method for treating the same



Sept@ 28, 1948.

FOR

Fil

v C. V. LITTO GARBURIZED FI A N Nov.

N ENT AND METHOD G THE SAME 25, m42

wooo? IOR' www/30s `BYA ' ATTO EY Patented Sept. 28, 1948 CARBURIZDFILAMENT AND METHOD FOR TREATING THE SAME A Charles V. Litton, RedwoodCity, Calif., assignor to Federal Telephone and Radio Corporation,Newark, N. J., a corporation of Delaware This invention relates toimprovements in carburized filaments and methods for treating the same,and more particularly to an improved method for reducing the brittlenessof carburized tungsten laments.

An object of this inventionY is to provide a pliable carburized tungstenfilament.

Another object of this invention is to provide a method for reducing thebrittleness of any carburlzed wire.

Still another, more comprehensive object of this invention is directedto a method for restraightening carburized tungsten laments.

Further objects and advantages of the present invention will becomeapparent from the following description of a preferred method,illustrated by way of example in the accompanying drawing, in which:

Fig. l is a cross-section of' an untreated carburized tungsten wire; and

Fig. 2 is ya similar cross-section of a tungsten Wire treated inaccordance with the method of this invention.

The carburization of tungsten or thoriated tungsten filaments is a stepwhich has become common practice in the filament art. After the lamentshave been formed they are heated in a carburizing atmosphere resultingin a thin layer of carburized tungsten surrounding an inner core ofrelatively pure metal. In most instances, however, the carburizationprocess tends to distort the filaments so that a re-straightening of thesame is necessary. Before such restraightening process the filamentshave a tendency to break since they have become brittle. The reason forthis brittleness and breakage, apparently, is that small cracks andfissures are formed in the outer carburized layer which extend throughthis layer and terminate on the virgin meta] beneath. ,Cold bending ofthe Wire results in high local stresses at the roots of the Y cracks,the stresses on the core reaching limiting values suiiicient to causebreaking thereof, and accordingly, a breaking of the entire filament.Fig. l illustrates, in a greatly enlarged cross-section, such a filamenthaving an inner core I and a carburized layer l2. The cracks I4appearing in the carburized layer I 2 are shown as extending into thecore Ill, particularly where the Wire is sharply bent resulting, incertain instances, in a through crack such as indicated at I6.

According to the method of the present invention the effectivebrittleness of the filament is reduced and breakage is prevented byflashing or otherwise heating the carburized iilament in a 23, 1942,Serial No. 466,567 1 claim. (C1. 25o-27.5)

medium inert to the carburized filament, for example, a gas such ashydrogen or helium, and bending the iilament into shape while the sameis still hot and is still in the inert medium. This heating of thefilament makes the inner core too ductile to break and the moving of thewire resulting from the re-straightening of the iilament while h'otresults in shearing stresses which tend to separate the core from itsouter carburized sheath. The net result of this lamellar fracture isthat stresses resulting from the radial fractures in the carburizedlayer are no longer localized and core fracture liability is accordinglyreduced. Additionally, the hydrogen flash after carburization appears toalter the crystal structure of the carbide layer and, after bending,viewed under the microscope the layer shows a very marked iine grainfracture pattern, the whole surface being crazed It might be said thatin the case of the treatment according t0 the present invention, thecarbide llayer acts as a true coating, whereas in the casev of untreatedwire it is merely a fractured surface applying stresses to the innercore at relatively few cleavage points. The minute "craaing accordinglyprevents localization of stresses at a few selected points, and henceunder distortion, such as bending, the core in addition to now beingmore ductile is not subjected to as high localized stresses.

A filament which has been treated in accordance with the presentinvention is illustrated by way of example in Fig. 2. It shows an innercore 20 of substantially virgin metal surrounded by an outer tungstencarbide layer 22 filled with a relatively large number of minute cracks24. The lamellar separation between. the outer layer 22 and the core 20is indicated on. an exaggerated scale at 26. This separation between theouter layer and core also prevents the cracks 24 from extending into theinner core 201, in contrast to the direct penetration of the cracks, asillustrated in Fig. 1.

While the principles of this invention are particularly useful inconnection with the formation of carburized tungsten iilaments, and morespecifically, the step of 1re-straightening suoli filaments, it isapparent that the method in its produced aspect is applicable forreducing the brittleness of any carburized wire, whether used for afilament or for some other purpose.

1 Accordingly, while I have described above the principles of myinvention in connection with a certain specific application, it is to beclearly understood that this description is made only by -surrounded by.an outer layer of thoriated tungsten carbide havinga crazed. surfaceand no connection other than surface contact connection With said core.

CHARLES V. LITTON.

REFERENCES CITED The fllwing referencesareof record ln the file of thispatent:

UNITED STATES PATENTS Number Name Date 716,311 Thomson Dec. ;1'6,. 19021,230,869 Coolidge June 26, 1917 Lederer July 20, 1926 Number NumberName Date Gero Mar. 27, 1928 Elsey Mar. 29, 1932 Elsey v June 7, 1932Iredeu Jan. 3, 1933 Iredell May 16, 1933 `Dester -....'A1ig."25, 1936Fonda Feb. 2, 1937 Smith Oct. 5, 1937 Taylor May 5, 1942 Peters Dec. 15,1942 FOREIGN PATENTS 'Country Date Switzerland June 1, 1929 Germany Oct.1, 1909 v l OTHER. REFERENCES The Principles of Physical Metallurgy,Doan 20 and Mahlo, 1941, page 131.

Metals Handbook, 1939 edition, page .268.

